Repairing amplifiers is like a box of chocolates. You never know.....
The tiny photoflash caps might have been okay, but why take the chance? The ugly green block is a 12-Volt transformer that was added to allow use of the cheaper 17KV6 tube in place of the rare and expensive 6KV6.
Yes, there are other compatible types that plug right in and run. They are all have smaller power ratings. This is the one odd detail that we won't be changing. The 17KV6 is still cheaper, and available from a variety of sources.
One important change is to add fixed bias to the control grids of the tubes. First the lug for pin 2 is lifted (gently) from ground. A bypass cap and return resistor both go from the lug to ground, and a wire goes from this right-hand socket to the same pin on the left-hand socket.
The real action is on the left-hand tube socket. It gets the same resistor, cap and the other end of the brown wire. It also gets the rectifier and filter that borrows 17 Volts from the tubes' heater supply. Thought this might be too much, but it worked out about right.
So, why two rectifier diodes here? There is of course the one half-wave rectifier diode between the socket's heater-voltage pin and the negative side of the filter cap. And there is a second diode between that spot and pin 2 of the tube socket. This blocking diode prevents the filter cap from being charged up with negative voltage from the tube's grid on modulation peaks. Just smooths things out.
The original relay had a 110-Volt DC coil, so a tube could be used to sense the radio's RF and activate the relay. The relay is a 45 (or more) year-old mechanical part. No good reason to leave it in there. We use a 12-Volt DC relay.
The socket for the now-unused keying tube is a handy spot to build a half-wave voltage doubler circuit fed from the 6.3-Volt AC heater winding.
Our everyday keying circuit is more sensitive than the keying tube and less likely to chatter the relay.
The new relay has a coil added between it and the tie strip where the original input-matching trimmer cap is mounted. The coil and trimmer cap together work a lot better than the original setup with the capacitor alone.
The scabbed-in 12-Volt transformer is ugly, but does what it's there to do. With any luck, this amplifier will have a lot more miles to go down the road before the next time it needs maintenance.
One last word about the disc caps that the factory connected from the power cord to the chassis, one on each power-cord wire. Lose the originals. They are a potential shock hazard. Never mind why.
The correct caps to replace them will be covered in hieroglyphic safety-approval symbols like these.
They're just less likely to become a short between the line cord and chassis if a lightning surge makes its way into the house when the utility's poles get struck.
73
The tiny photoflash caps might have been okay, but why take the chance? The ugly green block is a 12-Volt transformer that was added to allow use of the cheaper 17KV6 tube in place of the rare and expensive 6KV6.
Yes, there are other compatible types that plug right in and run. They are all have smaller power ratings. This is the one odd detail that we won't be changing. The 17KV6 is still cheaper, and available from a variety of sources.
One important change is to add fixed bias to the control grids of the tubes. First the lug for pin 2 is lifted (gently) from ground. A bypass cap and return resistor both go from the lug to ground, and a wire goes from this right-hand socket to the same pin on the left-hand socket.
The real action is on the left-hand tube socket. It gets the same resistor, cap and the other end of the brown wire. It also gets the rectifier and filter that borrows 17 Volts from the tubes' heater supply. Thought this might be too much, but it worked out about right.
So, why two rectifier diodes here? There is of course the one half-wave rectifier diode between the socket's heater-voltage pin and the negative side of the filter cap. And there is a second diode between that spot and pin 2 of the tube socket. This blocking diode prevents the filter cap from being charged up with negative voltage from the tube's grid on modulation peaks. Just smooths things out.
The original relay had a 110-Volt DC coil, so a tube could be used to sense the radio's RF and activate the relay. The relay is a 45 (or more) year-old mechanical part. No good reason to leave it in there. We use a 12-Volt DC relay.
The socket for the now-unused keying tube is a handy spot to build a half-wave voltage doubler circuit fed from the 6.3-Volt AC heater winding.
Our everyday keying circuit is more sensitive than the keying tube and less likely to chatter the relay.
The new relay has a coil added between it and the tie strip where the original input-matching trimmer cap is mounted. The coil and trimmer cap together work a lot better than the original setup with the capacitor alone.
The scabbed-in 12-Volt transformer is ugly, but does what it's there to do. With any luck, this amplifier will have a lot more miles to go down the road before the next time it needs maintenance.
One last word about the disc caps that the factory connected from the power cord to the chassis, one on each power-cord wire. Lose the originals. They are a potential shock hazard. Never mind why.
The correct caps to replace them will be covered in hieroglyphic safety-approval symbols like these.
They're just less likely to become a short between the line cord and chassis if a lightning surge makes its way into the house when the utility's poles get struck.
73
